Climate Change Scenarios in Southern California. Robert J. Allen University of California, Riverside Department of Earth Sciences

Transcription

1 Climate Change Scenarios in Southern California Robert J. Allen University of California, Riverside Department of Earth Sciences

2 Overview Climatology of Southern California Temperature and precipitation Importance of mid-latitude cyclones and ENSO Climate Models & GHG Emission Scenarios Coupled Model Intercomparison Project phase 3 (CMIP3) Bias Corrected Spatially Downscaled CMIP3 (BCSD-CMIP3) North American Regional Climate Change Assessment Program (NARCCAP) CMIP5 SoCal Climate Change by 2100: Highly significant, robust projection 2-4 C warming Less certain projection decreased precipitation, runoff, streamflow, soil moisture For medium and medium-high emissions scenarios, precipitation reduction becomes significant at 5-10%. Why models may underestimate this decrease

3 Climatology of Southern California Los Angeles San Diego Mediterranean climate hot, dry summers and mild, relatively wet winters Most precipitation occurs during winter/early spring due to largescale mid-latitude cyclones propagating westward from the Pacific Ocean

4 Importance of Midlatitude Cyclones/Storm Tracks JFM AMJ Most precipitation in SoCal comes from mid-latitude cyclones, especially during winter/late spring JAS OND Climate Prediction Center

5 ~40% more rain during an El Nino year relative to a neutral year and ~50% more rain relative to a La Nina year.

6 ENSO and Storm Tracks El Nino Southward shift of storm track from northern to southern part of US (including Southern California) more precipitation Climate Prediction Center

7 How Does ENSO Change With Warming? Most models project a weak shift towards an El Nino-like background state, but large differences in amplitude of ENSO variability in the future. How ENSO will change in the future is not well-known. IPCC, 2007

8 How do Storm Tracks Change? Storm tracks move poleward in response to greenhouse gases (NOTE: poleward shift in NH has already been observed ~0.4 per decade) Ulbrich et al., 2008

9 Climate Models CMIP3 archive 25 atmosphere-ocean global climate models (AOGCM) w/ spatial resolution ~200 km. BCSD-CMIP3 CMIP3 models are spatially downscaled using observations to ~12 km resolution. ~38 climate projections each w/ SRESB1, A1B and A2 from NARCCAP 6 regional climate models (50 km resolution) nested within several different CMIP3 GCMs. 10 total projections based on SRESA2 from CMIP5 archive ~45 projections from 28 AOGCMs/Earth System Models at ~100 km spatial resolution using RCP8.5 from

10 Future Greenhouse Gas Emission Scenarios CMIP3 Scenarios CMIP5 Scenarios 1,000 RCP RCP RCP 6.0 RCP SRESB1 low emission scenario (500 ppm CO 2 by 2100) 2. SRESA1B medium emissions (720 ppm by 2100) 3. SRESA2 medium-high emissions (850 ppm by 2100). 4. RCP8.5 high emissions scenario for CMIP5 (950 ppm by 2100) Nakicenovic and Swart, 2000; Moss et al. Nature 2010

11 BCSD-CMIP3 Western US Temperature Change General warming of western US, with minimum warming along west coast

12 BCSD-CMIP3 Western US Precipitation Change General increases in precipitation to the north and (weaker) decreases to the south.

13 BCSD-CMIP3 SoCal Precipitation Change Largest precipitation reductions occur 1. along the coast; 2. in the southwest corner

14 BCSD-CMIP3 SoCal DT Seasonal Box-Plots Highly significant warming of C by and C by Maximum warming in JJA/SON.

15 BCSD-CMIP3 SoCal DP The high emission scenario (and A1B) shows significant reductions of 5-10% in SoCal precipitation for both time periods. Maximum decrease in MAM of -29% for A2 by Increase in JJA precipitation (strengthening of NA monsoon?)

16 BCSD-CMIP3 SoCal DRunoff The high emission scenario (and A1B) shows significant reductions in SoCal runoff of -13% by Maximum decrease in MAM of -33% (SRESA2).

17 BCSD-CMIP3 SoCal DSoil Moisture The high emission scenarios (and A1B) shows significant reductions in SoCal soil moisture of -3.6% by Maximum decrease in MAM.

18 NARCCAP SoCal Climate Change Highly significant Annual mean warming of 2.1 C (JJA/SON maximum). Not Significant Decreases in annual mean precipitation of -5% (-18% MAM). DPrecipitation is similar to BCSD-CMIP3, but with more uncertainty.

19 NARCCAP: CCSM vs. GFDL DPrecipitation -13.9% -19.0% -14.4% +1.8% +0.6% -6.0% Large differences between RCMs using CCSM boundary conditions versus GFDL. Reinforces the notion large-scale atmospheric circulation changes are likely most important.

20 DTemperature DPrecipitation CMIP5 RCP85 Ensemble Mean Changes Similar spatial pattern (for both T and Precip) to BCSD-CMIP3 ensemble mean.

21 CMIP5 RCP85 Seasonal SoCal Changes Highly significant annual mean warming of 2.4 C (4.3 C) by ( ). Significant decreases in annual mean precipitation of -5% for both time periods. DPrecipitation is similar, but weaker than BCSD-CMIP3 (JJA increase also lacking).

22 CMIP3 Models Underestimate Poleward Displacement of Circulation MMC OLR P-E P-E Suggests models may underestimate future decreases in SoCal precipitation? Johanson+Fu, 2009

23 Conclusions A wide range of climate model data was evaluated to assess future SoCal climate change. Robust projection: Warming of C by mid-century. Warming of C by end of century. Precipitation changes are less certain, but the multi-model mean shows decreased precipitation. 5-10% reductions by end of century. Similar reductions in other hydrological variables (e.g., streamflow) Generally, larger magnitude change for higher GHG emission scenarios (e.g., SRESA2 and RCP85). Models may underestimate the precip decrease in SoCal, but uncertainties (e.g., ENSO) and large natural climate variability remain.

24 The End Thanks!!

25 Palm Springs Colorado River above Imperial Dam

26 Large-Scale DT and DP: CMIP3 IPCC, 2007

27 Additional Hydrological Changes IPCC, 2007

28 DJF Cyclone Tracks 2002/2003 Bengtsson et al., 2006

29 ENSO Versus Global Warming Lu et al, 2008

30 Importance of Midlatitude Cyclones/Storm Tracks Most precipitation in SoCal comes from mid-latitude cyclones, especially during winter/late spring Hodges et al., 2011

31 How do Storm Tracks Change? Storm tracks move poleward in response to greenhouse gases (NOTE: poleward shift has already been observed (~0.4 per decade) Yin GRL, 2005

32 htt :// is.usbr. ov/streamflow_ ro ections/ BCSD-CMIP3-Based SoCal Streamflow Five locations in Southern California: 1. Velyermo; 2. Keenbrook; 3. Arrow Head Springs; 4. Palm Springs; and 5. Colorado River above Imperial Dam. Streamflow based on Variable Infiltration Capacity hydrological model.

33 SoCal DStreamflow (A2 Scenario) Colorado River Palm Springs Valyermo Arrowhead Springs Keenbrook Considerable yearto-year variability, but significant decreases at all locations by end of century.

34 Map of Annual Mean SoCal Precip

35 Role of El Nino-Southern Oscillation Sea Level Pressure Surface Temperature Precipitation El Nino is associated with anomalous wet conditions in SoCal IPCC, 2007

36 Importance of non-ghgs in Driving Poleward Displacement of Circulation Other climate forcing agents besides GHGs exist. Only ~50% of CMIP3 models included timevarying black carbon and ozone. SRES did not specify future concentrations. Further supports model underestimation Allen et al., Nature 2012